The high cost of transmission paths is the reason for looking for compression means that allow signals from several different sources to be sent over the same path. Thus, techniques have been developed for concentrating speech-originating signals and data on the same line. Some of these techniques use the silent intervals of the speech for transmitting data; others remove artificially a frequency band in the speech signal spectrum to insert the data. The means for implementing these techniques do not permit presently obtaining a satisfactory quality/price ratio.
On the other hand, the cost of digital signal processing devices is at present relatively low, and the tendency is to develop entirely digital speech compression techniques. Thus, to transmit simultaneously several speech signals over a telephone line, vocoders (voice coders) with a low bit rate (2400 to 4800 bps) have been used with their outputs being digitally multiplexed over the same telephone line equipped with modems operating at 4800 or 9600 bps. However, the low bit rate vocoders used are often based on techniques requiring measurement of the fundamental (pitch) frequency. The hazards of this measurement considerably reduce the quality of the transmitted speech.